1. Field
The present teachings relate to sensing current. In particular, the present teachings relate to using a comparator for sensing the amplitude of current flowing out of a circuit.
2. Description of Related Art
Current sensing can be carried out in a variety of ways. In a first approach, the current is propagated through a resistor having a known resistance value and the resulting voltage drop across the resistor is measured in order to determine the amplitude of the current flowing through the resistor. As can be appreciated this approach involves undesirable power wastage in the resistor, thereby leading to a reduction in the overall amount of power that can be provided to a load circuit.
In a second approach, an operational amplifier is configured to generate a sense signal that is proportional to the value of a current flowing through a current carrying line. While the operational amplifier approach addresses the undesired output power reduction problem associated with a current sensing resistor, this second approach also suffers from several shortcomings.
To elaborate upon this aspect, attention is drawn to FIG. 1, which shows an operational amplifier 210 configured to detect the amplitude of current flowing out a circuit 205 and into a load circuit 241. As shown, operational amplifier 210 is configured for sensing the amplitude of current flowing out of circuit 205 and into load circuit 241. Specifically, in this particular configuration, an output node 230 of circuit 205 is coupled to a negative input terminal of operational amplifier 210, and a second node (junction node 231) is coupled into the positive input terminal of operational amplifier 210. An output terminal of operational amplifier 210 is coupled to a gate terminal of an nFET 218 so as to provide feedback control to operational amplifier 210, by controlling the drain-source current flow in nFET 218. This operation will be explained below in further detail.
Junction node 231 is located between a circuit 215 and a variable impedance circuit composed of nFET 218 and resistor 219. Circuit 215, which may be used for carrying out a control function upon circuit 205 (via line 216), also constitutes an impedance that provides for current flow into nFET 218, thereby resulting in a voltage potential at junction 231.
A sense signal is derived at node 233 and coupled into control circuit 225 (via line 232). The sense signal is indicative of the amplitude of current flowing into load circuit 241 from circuit 205, and is used by control circuit 225 to generate suitable control signals for controlling various elements (not shown) inside circuit 205, thereby controllably modifying the amplitude of current flowing out of circuit 205 into load circuit 241.
As mentioned above, the use of operational amplifier 210 does indeed address the undesired output power reduction problem associated with power dissipation in a current sensing resistor. However, the operational amplifier solution also suffers from several handicaps.
The first handicap pertains to the bandwidth requirement of operational amplifier 210. Typically, for effective operation of the feedback circuit configuration shown in FIG. 1, the bandwidth of operational amplifier 210 has to be selected to be at least one order of magnitude higher than the bandwidth of the AC signal present at junction node 230 in order to produce a desired gain. For example, if the frequency of the AC signal provided to the non-inverting terminal of operational amplifier 210 (from junction node 230) is around 25 MHz, operational amplifier 210 has to be selected to have a bandwidth of around 250 MHz (i.e., a decade multiple of the input AC frequency). This large bandwidth requirement places an undesirable limitation on the type of operational amplifier that can be used in current sensing circuit 220, consequently curtailing choices of operational amplifiers to a limited sub-set of devices amongst a plurality of available devices.
The second handicap pertains to power consumption. The large bandwidth of operational amplifier 210 leads to an increase in power consumption, thereby leading to an undesirable increase in demand upon the power supply. The increased power consumption also leads to other problems such as thermal issues and increased real estate requirements on a substrate of an integrated circuit, for example.
The third handicap pertains to stability issues associated with large bandwidth operational amplifiers. The large bandwidth of operational amplifier 210 can lead to unstable circuit performance, especially when the gain of the circuit is increased beyond a certain level. Consequently, the selection of the type of components used, as well as circuit design and layout aspects, become critical issues.
Several other handicaps associated with large bandwidth operational amplifiers include factors such as cost, availability, and packaging constraints.